Can You Just Take a Look at this Legacy Code?

As a programmer there are a number of books which people will tell you are must read books for any professional – which do change over time as programming techniques evolve. However the books are fairly consistent in that they all tend to be written from the point of view of a green field system, starting from first principles, ensuring you build a maintainable system.

But is that realistic? You might be lucky and get in at the beginning of a brand new startup, or you could land a job at a consultancy where you’re always writing bespoke code, but for most programmers an awful lot of their career will be dealing with the joys of legacy code.

It may be that you come into an established company with many years of development and thousands of lines of code debt and changing technologies.

Alternatively you could be handed the thing programmers often dread the “business developed application” – often these are mired in corporate politics as well, with strained relations between the business area that developed the application and the IT department. Indeed in one company I worked for there was a semi-secret development team in one part of the business formed as a result of the IT department saying no one too many times! In most cases these business developed applications are produced by people whose strength is in understanding how the business works, but are inexperienced as developers, which often produces a double hit of problems in that the business logic is usually poorly documented, and the code is also of poor quality.

Other examples I’ve come across are prototype systems that have almost accidentally ended up as critical systems, and something that happens surprisingly often is a big company takes on responsibility for a third party product either because they don’t want to upgrade to a supported version, or because the third party company is abandoning a product altogether.

The common factor in all of these is that you’re taking on a codebase that is less than ideal, so all these coding books that assume you’re starting from scratch aren’t overly useful. All the benefits of test driven development protecting you when you make changes are really not much good when you have incomplete or totally missing tests. It’s incredibly difficult to find your way around a badly structured code base if you’re used to textbook structures and accurate documentation.

What do you do? Edit and pray it works? Rewrite the whole system from scratch?

All of which brings me back to where I started, and the excellent Working Effectively with Legacy Code by Michael Feathers. The book starts from the entirely pragmatic position that you are going to be working on dodgy code a lot of the time, and if you don’t want to make it worse you need to get it sorted out. It is also realistic in that it gives you techniques to gradually improve the code as a business will rarely be able to spare the time and resources to totally rewrite something.

The really basic concept around which a lot of the more complicated techniques are built is that whilst you can’t bring all of a codebase under test immediately you can grow islands of properly tested code within the codebase that gradually spread out as you work on other parts of the codebase over time. To create these islands you need to separate them from the rest of the codebase, which is where a lot of the complexity comes from, but Feathers offers a variety of different techniques for making those separations. The ultimate aim is that as much of your legacy codebase is brought under test, and the codebase as far as possible conforms to modern principles like DRY and SOLID, whilst at the same time allows you to produce the changes and improvements your users or customers are demanding to the legacy code.

I hesitate to say that any programming book is an essential read, but if like most programmers you’re faced with a chaotic legacy codebase Working Effectively with Legacy Code is a book that certainly gives you a lot of practical advice of how to make things better.